U.S. patent application number 11/965620 was filed with the patent office on 2009-07-02 for lead frame die attach paddles with sloped walls and backside grooves suitable for leadless packages.
Invention is credited to Andrew Abarrientos, Romolo Bactasa, Omar A. Janducayan.
Application Number | 20090166826 11/965620 |
Document ID | / |
Family ID | 40797134 |
Filed Date | 2009-07-02 |
United States Patent
Application |
20090166826 |
Kind Code |
A1 |
Janducayan; Omar A. ; et
al. |
July 2, 2009 |
LEAD FRAME DIE ATTACH PADDLES WITH SLOPED WALLS AND BACKSIDE
GROOVES SUITABLE FOR LEADLESS PACKAGES
Abstract
Disclosed are die paddle structures for leadframes and methods
of attaching die to the die paddles. An exemplary die paddle
comprises a sloped wall disposed around an attachment area for a
die, where the sloped wall has an obtuse angle of inclination with
respect to the attachment area. In one exemplary die-attachment
process, solder material is disposed on the attachment area and/or
the metalized back surface of a die, the die is placed over the
attachment area and substantially within the opening defined by the
sloped wall, and the solder is reflowed while the die is allowed to
float over the reflowed solder free of external forces from a
die-placement tool and to align itself to the sloped wall. Die
paddles and attachment methods of the invention reduce the
alignment tolerances needed to place the die.
Inventors: |
Janducayan; Omar A.; (Lapu
Lapu City, PH) ; Bactasa; Romolo; (Singapore, SG)
; Abarrientos; Andrew; (Lapu Lapu City, PH) |
Correspondence
Address: |
TOWNSEND AND TOWNSEND AND CREW, LLP
TWO EMBARCADERO CENTER, EIGHTH FLOOR
SAN FRANCISCO
CA
94111-3834
US
|
Family ID: |
40797134 |
Appl. No.: |
11/965620 |
Filed: |
December 27, 2007 |
Current U.S.
Class: |
257/676 ;
257/E21.51; 257/E23.04; 438/123 |
Current CPC
Class: |
H01L 24/48 20130101;
H01L 2224/2919 20130101; H01L 2924/01074 20130101; H01L 2924/01046
20130101; H01L 2224/83101 20130101; H01L 2224/48247 20130101; H01L
2924/00014 20130101; H01L 2224/48091 20130101; H01L 24/32 20130101;
H01L 2224/32257 20130101; H01L 2224/83193 20130101; H01L 2224/86
20130101; H01L 2924/01014 20130101; H01L 2924/01043 20130101; H01L
24/49 20130101; H01L 2924/13091 20130101; H01L 2924/181 20130101;
H01L 2224/8314 20130101; H01L 2924/01028 20130101; H01L 2924/01079
20130101; H01L 2224/83385 20130101; H01L 2224/83801 20130101; H01L
23/49503 20130101; H01L 2924/01082 20130101; H01L 2224/83855
20130101; H01L 2924/07802 20130101; H01L 24/29 20130101; H01L
2224/26175 20130101; H01L 2224/83143 20130101; H01L 24/75 20130101;
H01L 2224/49 20130101; H01L 2224/27013 20130101; H01L 2224/50
20130101; H01L 24/73 20130101; H01L 2924/01065 20130101; H01L
2224/291 20130101; H01L 2224/73265 20130101; H01L 24/83 20130101;
H01L 2224/83191 20130101; H01L 2224/83815 20130101; H01L 2924/1305
20130101; H01L 24/34 20130101; H01L 2224/45014 20130101; H01L
2224/48257 20130101; H01L 2224/92247 20130101; H01L 24/33 20130101;
H01L 2924/01033 20130101; H01L 2924/01029 20130101; H01L 23/49513
20130101; H01L 2224/83051 20130101; H01L 2224/83192 20130101; H01L
2224/32245 20130101; H01L 2924/014 20130101; H01L 2224/48472
20130101; H01L 2924/0781 20130101; H01L 2224/48091 20130101; H01L
2924/00014 20130101; H01L 2224/48091 20130101; H01L 2924/00014
20130101; H01L 2224/48472 20130101; H01L 2224/48247 20130101; H01L
2924/00 20130101; H01L 2224/83192 20130101; H01L 2224/32245
20130101; H01L 2224/48247 20130101; H01L 2924/13091 20130101; H01L
2224/48257 20130101; H01L 2924/13091 20130101; H01L 2224/48247
20130101; H01L 2924/00012 20130101; H01L 2224/73265 20130101; H01L
2224/32245 20130101; H01L 2224/48247 20130101; H01L 2924/00012
20130101; H01L 2224/73265 20130101; H01L 2224/32245 20130101; H01L
2224/48257 20130101; H01L 2924/00012 20130101; H01L 2224/83192
20130101; H01L 2224/83101 20130101; H01L 2924/00 20130101; H01L
2224/48247 20130101; H01L 2924/00012 20130101; H01L 2224/83191
20130101; H01L 2224/83101 20130101; H01L 2924/00 20130101; H01L
2224/48247 20130101; H01L 2924/00012 20130101; H01L 2224/92247
20130101; H01L 2224/73265 20130101; H01L 2224/32245 20130101; H01L
2224/48247 20130101; H01L 2924/00012 20130101; H01L 2224/92247
20130101; H01L 2224/73265 20130101; H01L 2224/32245 20130101; H01L
2224/48257 20130101; H01L 2924/00 20130101; H01L 2224/48472
20130101; H01L 2224/48091 20130101; H01L 2924/00 20130101; H01L
2224/291 20130101; H01L 2924/014 20130101; H01L 2224/2919 20130101;
H01L 2924/0665 20130101; H01L 2924/1305 20130101; H01L 2924/00
20130101; H01L 2924/00014 20130101; H01L 2224/45099 20130101; H01L
2924/00014 20130101; H01L 2224/05599 20130101; H01L 2924/181
20130101; H01L 2924/00012 20130101; H01L 2924/00014 20130101; H01L
2224/37099 20130101; H01L 2224/83801 20130101; H01L 2924/00014
20130101; H01L 2224/50 20130101; H01L 2924/00014 20130101; H01L
2224/86 20130101; H01L 2924/00014 20130101; H01L 2924/00014
20130101; H01L 2224/45014 20130101; H01L 2924/206 20130101; H01L
2224/92247 20130101; H01L 2224/73265 20130101; H01L 2224/32245
20130101; H01L 2224/48257 20130101; H01L 2924/00012 20130101 |
Class at
Publication: |
257/676 ;
438/123; 257/E23.04; 257/E21.51 |
International
Class: |
H01L 21/60 20060101
H01L021/60; H01L 23/495 20060101 H01L023/495 |
Claims
1. A leadframe for holding a semiconductor die and making
electrical connections thereto, the leadframe comprising: a die
paddle having a first surface, a second surface, a perimeter, an
attachment area located on the first surface and within the
perimeter to receive a semiconductor die, and a first thickness
between the first and second surfaces; a sloped wall disposed in
the first surface of the die paddle and surrounding the attachment
area, the sloped wall having an angle of inclination with respect
to the attachment area of more than 90 degrees; and a plurality of
conductive regions disposed along at least a portion of the
perimeter of the die paddle and spaced apart from said
perimeter.
2. The leadframe of claim 1 wherein the sloped wall has a height
with respect to the attachment area of at least 65 micron.
3. The leadframe of claim 1 wherein the sloped wall has a height
with respect to the attachment area of at least 130 microns.
4. The leadframe of claim 1 wherein the sloped wall has a height
with respect to the attachment area of at least 155 microns.
5. The leadframe of claim 1 wherein the sloped wall has a height
with respect to the attachment area that is greater than 60% of the
first thickness.
6. The leadframe of claim 1 further comprising a groove formed in
the second surface of the die paddle and opposite to at least a
portion of the sloped wall.
7. The leadframe of claim 1 further comprising a ridge disposed in
the first surface of the die paddle and surrounding the attachment
area, the ridge having an inner wall facing toward the attachment
area and an outer wall facing the die paddle perimeter, at least a
portion of the sloped wall being coextensive with at least a
portion of the sloped wall.
8. The leadframe of claim 7 further comprising a groove formed in
the second surface of the die paddle and opposite to at least a
portion of the ridge.
9. The leadframe of claim 1 wherein the angle of inclination of the
sloped wall is equal to or less than about 135 degrees.
10. The leadframe of claim 1 wherein the angle of inclination of
the sloped wall lies in a range that spans from about 95 degrees to
about 120 degrees.
11. A semiconductor die package comprising: a die paddle having a
first surface, a second surface, a perimeter, an attachment area
located within the perimeter to receive a semiconductor die, and a
first thickness between the first and second surfaces; a sloped
wall disposed in the first surface of the die paddle and
surrounding the attachment area and defining an opening to the
attachment area, the sloped wall having an angle of inclination
with respect to the attachment area of more than 90 degrees, the
sloped wall having a height with respect to the attachment area of
at least 65 microns; a semiconductor die having a first surface, a
second surface, and a second thickness between its surfaces, the
semiconductor die being disposed over the attachment area and
within the opening; and an adhesive layer disposed between the
second surface of the semiconductor die and the attachment area,
the solder layer having a third thickness.
12. The semiconductor die package of claim 11 wherein the sloped
wall has a height with respect to the attachment area of at least
130 microns.
13. The semiconductor die package of claim 11 wherein the height of
the sloped wall is at least equal to the third thickness plus
one-half of the second thickness less 20 microns.
14. The semiconductor die package of claim 11 wherein the height of
the sloped wall is at least equal to the third thickness plus the
second thickness less 20 microns.
15. The semiconductor die package of claim 11 wherein the adhesive
layer comprises solder and has a thickness of at least 50
microns.
16. The semiconductor die package of claim 11 wherein the adhesive
layer comprises solder and has a thickness of at least 75
microns.
17. The semiconductor die package of claim 11 further comprising a
groove formed in the second surface of the die paddle and opposite
to at least a portion of the sloped wall.
18. The semiconductor die package of claim 11 further comprising a
ridge disposed in the first surface of the die paddle and
surrounding the attachment area, the ridge having an inner wall
facing toward the attachment area and an outer wall facing the die
paddle perimeter, at least a portion of the sloped wall being
coextensive with at least a portion of the sloped wall.
19. The semiconductor die package of claim 18 further comprising a
groove formed in the second surface of the die paddle and opposite
to at least a portion of the ridge.
20. The leadframe of claim 11 wherein the angle of inclination of
the sloped wall is equal to or less than about 135 degrees.
21. The leadframe of claim 11 wherein the angle of inclination of
the sloped wall lies in a range that spans from about 95 degrees to
about 120 degrees.
22. A system comprising a substrate and the semiconductor die
package of claim 11 attached to the substrate.
23. A method of attaching a semiconductor die to a die paddle
comprising: disposing a quantity of solder material on one or both
of a surface of a semiconductor die and an attachment area of a die
paddle, the attachment area having a first thickness and being
substantially surrounded by a sloped wall, the sloped wall having
an angle of inclination with respect to the attachment area of more
than 90 degrees; placing the semiconductor die over the attachment
area so that at least a major portion of the die overlays the
attachment area, and so that solder material is located between the
die and the attachment area; applying heat to the solder material
to place it in a liquid state; and allowing the semiconductor die
to float on the solder for at least a portion of the time the
solder is in its liquid state.
24. The method of claim 23 further comprising allowing the solder
to solidify to form a finished solder layer, and wherein the
quantity of solder material is dispensed in a preset amount to
provide a finished solder layer having a target thickness in the
range of approximately 25 microns to 200 microns.
25. The method of claim 24 wherein the quantity of solder material
is dispensed in a preset amount so that the bond line thickness is
within 5 microns of the target thickness when the method is
repeated with a second instance of the die paddle and a second
instance of the die.
26. A method of attaching a semiconductor die to a die paddle
comprising: disposing a quantity of adhesive material in a liquid
state on one or both of a surface of a semiconductor die and an
attachment area of a die paddle, the attachment area having a first
thickness and being substantially surrounded by a sloped wall, the
sloped wall having an angle of inclination with respect to the
attachment area of more than 90 degrees; and placing the
semiconductor die over the attachment area so that at least a major
portion of the die overlays the attachment area, and so that
adhesive material is located between the die and the attachment
area.
27. The method of claim 26 further comprising: applying a force in
the direction of gravity to the exposed surface of the die without
substantially restriction the die's horizontal movement relative to
the die paddle while the adhesive material is in a liquid
state.
28. The method of claim 27 wherein the force is applied by member
having a flat surface placed over the die with its flat surface in
contact with the die, and wherein the method further comprises at
least one of the following: applying vibrations to at least one of
the die paddle and the member; moving at least one of the die
paddle and the member.
Description
CROSS-REFERENCES TO RELATED APPLICATIONS
[0001] NOT APPLICABLE
BACKGROUND OF THE INVENTION
[0002] A leadframe is a thin layer of patterned metal that connects
the electrical pads on the surface of a semiconductor die to the
larger electrical terminals of a die package. A leadframe typically
comprises a large metal area, called a die paddle, to hold a
semiconductor die, and several electrical leads, with the inner
ends of the leads disposed about the perimeter of the die paddle.
Wire bonds, or the like, electrically couple the inner ends of the
leads to the pads of the die. The outer ends of the leads are
located away from the die padded and provide the electrical
terminals of the package. Typical leadframe have thicknesses
between 150 microns and 250 microns, and are made by stamping
and/or etching processes. Typically, hundreds of leadframes are
manufactured onto a common strip of metal, and are separated from
one another after semiconductor dice have been attached to the die
paddles and wire bonds have been made. Typically, the strip has
various indexing apertures disposed at each leadframe site that
enable the strip to be fed into and automatically aligned to the
die-attachment and wire-bonding equipment.
[0003] A body of plastic material is typically molded around the
die paddle and the leads, which may be done before or after the
leadframes are separated from the strip. In either case, an outer
frame of metal is provided in the leadframe to hold the leads and
die paddle in place during the above-described manufacturing steps.
Typically, the outer frame is made of the same sheet of metal as
the leads and paddle, and is connected to the outer ends of the
leads, and connected to the paddle by way of two or more metal
bridges. After the package is formed, the leadframe and package are
separated from the outer frame.
[0004] Efforts have been made to increase the efficiency of each
manufacturing step of the leadframe assembly. One current effort is
to use die-attachment equipment that simultaneously attaches
several semiconductor dice onto several leadframes, rather using
equipment that attaches one die at a time. However, as a
disadvantage, the multi-die attachment equipment cannot place dice
on the leadframes with the same degree of positional accuracy as
the single-die attachment equipment, or with the same degree of
control over the bond-line thickness (BLT), which is the thickness
of the adhesive disposed between the die paddle and leadframe. The
loss of accuracy appears to be due a combination of the following
factors: movement in the leadframe strip, mechanical play in the
equipment (which spans several die positioning tools), and changes
in the dimensions of the equipment and the leadframe strip caused
by temperature variations. Thus, the practical use of the more
efficient multi-die attachment equipment is substantially limited
to dice having medium- to large-sized pads, and to leadframes
having medium to large dimensions and leads.
BRIEF SUMMARY OF THE INVENTION
[0005] As part of making their invention, the inventors have
discovery new die paddle structures that, when coupled with new
die-attachment processes, enable the practical use of multi-die
attachment equipment on dice having small pads and leadframes
having small dimensions and small leads. Broadly stated, a die
paddle according to the present invention comprises a sloped wall
disposed around an attachment area for a die, where the sloped wall
has an obtuse angle of inclination with respect to the attachment
area. As one novel die-attachment process, solder material is
disposed on the attachment area and/or the metalized back surface
of a die, the die is placed over the attachment area and
substantially within the opening defined by the sloped wall, and
the solder is reflowed while the die is allowed to float over the
reflowed solder free of external forces from a die-placement tool
(e.g., detached from the die-placement tool). The surface tension
and metal-adhesion properties of the reflowed solder bring the
floating die into more precise alignment to the sloped wall. As
another novel die-attachment process, adhesive material is disposed
on the attachment area and/or the back surface of a die, the die is
placed over the attachment area and substantially within the
opening defined by the sloped wall, and the die is brought into
alignment with one or more of the following features: application
of light vertical pressure to the die by a pressure plate without
substantially restriction the die's horizontal movement relative to
the die paddle, application of vibrations to either or both of the
die paddle and the pressure plate, and movement of either or both
of the die paddle and the pressure plate. Accordingly, die may be
attached to die paddle according to the invention using both solder
and non-solder material.
[0006] In addition to the above advantageous effects, die paddle
structures according to the invention may be used to prevent
adhesive material, which includes solder and epoxies but not
limited thereto, from spreading beyond the defined die attachment
area, and thus preventing the adhesive material from interfering
with the operation of other components of the leadframe. As a
further advantageous effect, the bond line thickness (BLT) of a
layer of adhesive material located between the die and the paddle's
attachment area may be controlled by disposing a measured amount of
adhesive material on the back side of the die and/or on the die
attachment area. Controlling the BLT enables the thermal expansion
and conductivity characteristics of an assembled package to be
better controlled. In further embodiments of the present invention,
a die paddle further comprises a groove located in the paddle's
back surface and opposite to at least a portion of the sloped wall.
When a leadframe having this die paddle is incorporated into a
package with the paddle's back surface exposed, the package may be
mounted to a circuit board with the paddle's back surface adhered
to the board with adhesive material. The groove absorbs excess
adhesive material and prevents such material from spreading over
the surface of the board and interfering with the operation of
other components of the board. Additionally, a groove that is
partially or fully filled with adhesive material increases the
bonding strength and shear strength of the bond between the package
and the board.
[0007] These and other embodiments of the invention are described
in detail in the Detailed Description with reference to the
Figures. In the Figures, like numerals may reference like elements
and descriptions of some elements may not be repeated.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 shows a perspective top view of an exemplary
leadframe embodiment according to the invention.
[0009] FIG. 2 shows a cross-sectional view of an exemplary package
embodiment according to the invention that uses the exemplary
leadframe shown in FIG. 1 according to the invention.
[0010] FIG. 3 shows a plan view of the bottom surface of the
exemplary package embodiment shown in FIG. 1 according to the
invention.
[0011] FIG. 4 shows a cross-sectional view of the exemplary package
embodiment shown in FIG. 2 as mounted on a board according to the
invention.
[0012] FIGS. 5 and 6 show cross-sectional views of the exemplary
leadframe embodiment shown in FIG. 1 during a first exemplary
die-attachment process according to the invention.
[0013] FIGS. 7 and 8 show cross-sectional views of the exemplary
leadframe embodiment shown in FIG. 1 during a second exemplary
die-attachment process according to the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0014] FIG. 1 shows a perspective view of an exemplary embodiment
10 of a leadframe according to the invention for holding a
semiconductor die 5 and for making electrical connections to it.
Leadframe 10 comprises an outer frame 12, a die paddle 20 attached
to frame 12 by bridges 14, and a plurality of conductive regions 16
disposed around die paddle 20 and attached to outer frame 12, all
of which are typically metallic. These components may comprises a
base layer of copper (Cu) that is coated or alloyed with the
following order of metal sub-layers: nickel (Ni), palladium (Pd),
and gold (Au). Outer frame 12 and bridges 14 serve to hold the
relative positions of die paddle 20 and conductive regions 16 in
place during various processing steps of forming a package from
leadframe 10 and a semiconductor die. After a package has been
formed, outer frame 12 can be separated from conductive regions 16
and bridges 14 along cut planes 18 (shown by dashed lines) by
conventional cutting processes. Leadframe 10 may be incorporated
into a roll of leadframes (e.g., a hundred or more leadframes
arranged in serial form on a strip of conductive material which has
been stamped and/or etched to define the features of the frames),
which may include various indexing apertures for assisting the
transport of the leadframes through processing equipment and for
aligning the leadframes to the equipment.
[0015] Die paddle 20 has a first surface 21, a second surface 22, a
perimeter 24, an attachment area 26 disposed within perimeter 24 to
receive semiconductor die 5, and a thickness Tp between surfaces 21
and 22. Typical thicknesses of die paddles range between .about.150
microns (6 mils) and .about.250 microns (10 mils), which are the
typical thicknesses of leadframes, and can also be an thin as
.about.100 microns (.about.4 mils) and as thick as .about.500
microns (20 mils). A sloped wall 31 surrounds attachment area 26.
Sloped wall 31 may be provided by a ridge 30 that is formed in
first surface 21, with ridge 30 having an inner sloped wall as wall
31, and an outer sloped wall 32. Referring to FIG. 2, sloped wall
31 preferably has an angle of inclination .theta. with respect to
attachment area 26 of more than 90 degrees and less than about 135
degrees, and more preferably in a range that spans from about 95
degrees to about 120 degrees. Sloped wall 31 preferably has a
height H with respect to attachment area 26, and defines an opening
36 to attachment area 26 that has a width Wo that is larger than
the corresponding width Wa of attachment area 26 (the corresponding
widths being along the same dimensional direction). It may be
appreciated that sloped wall 31 may be provided by other
structures, such as a mesa, raised rim, or raised flange that
surrounds attachment area 26, thereby making area 26 into a recess
with sloped wall 31 as the recess' wall. It may also be appreciated
that two or more of the aforementioned structures (e.g., ridge,
mesa, rim, flange) may be used to provide sections of sloped wall
31 (for example, a mesa on one side of area 26, and a ridge on one
or more sides of area 26).
[0016] FIG. 2 shows a cross-sectional view of an exemplary
semiconductor die package 100 according to the invention employing
the exemplary die paddle 20 and leadframe 10 shown in FIG. 1. The
same reference numbers have been used in this figure (and in
subsequent figures) to reference previously-described elements and
features. In FIG. 2, the previously-described wall height H,
inclination angle .theta., and bottom paddle surface 22 can be more
clearly seen. In preferred uses of leadframe 10, a layer 40 of
adhesive (such as solder) is used to attach die 5 to attachment
area 26 with layer 40 having a controlled bond-line thickness (BLT)
Tb between die 5 to attachment area 26. The control can be provided
by disposing a measured amount of solder material on the backside
of die 5 and/or within opening 36, such as in the form of a paste
or one or more preformed bodies, placing die 5 over opening 36,
applying heat to the solder material to cause it to reflow, and
allowing die 5 to float freely over the reflowed solder (e.g., no
external pressure applied to the die) at least until the die aligns
its sides to the sides of sloped wall 31. At the present time,
typical values of bond-line thickness Tb range from about 10
microns (.about.0.4 mils) to about 200 microns (8 mils), with
values between about 50 microns (2 mils) and about 150 microns (6
mils) being preferred, and values between about 75 microns (3 mils)
and about 150 microns being more preferred. Larger amounts of
solder, and hence larger BLTs, facility the die's ability to align
itself to sloped wall 31 during solder reflow. The solder material
may be disposed by any conventional process that enables the
quantity to be dispensed in precise amounts, such as solder paste
dispensing, ribbon dispensing, and perform dispensing.
[0017] For very thin die, which have thicknesses of about 75
microns (3 mils) or less, the height H of sloped wall 31 preferably
has a value of 85 microns or more, and more preferably has a value
that is within about 20 microns of the combined thicknesses of the
die Tc and the desired bond-line thickness Tb (i.e., H=Tc+Tb.+-.20
microns). For a thin die with thickness Tc of 75 microns, this
preferred range of H would be .about.65 microns to .about.295
microns for target BLTs in the range of .about.10 microns to
.about.200 microns; would be .about.105 microns to .about.245
microns for target BLTs in the range of .about.50 microns to
.about.150 microns; and would be .about.130 microns to .about.245
microns for target BLTs in the range of .about.75 microns to
.about.150 microns. The relationship H=Tc+Tb.+-.20 microns may be
applied to thicker die having thicknesses in the range of 100
microns to 355 microns. Table I provides exemplary ranges of H for
sample chip thicknesses and exemplary bond line thickness
ranges.
TABLE-US-00001 TABLE I Wafer Bond Line Thickness (Tb) Ranges
Thickness (Tc) 25-200 microns 50-150 microns 75-150 microns 100
microns 105-320 microns 130-270 microns 155-270 microns 125 microns
130-345 microns 155-295 microns 180-295 microns 200 microns 205-420
microns 230-370 microns 255-370 microns 355 microns 360-575 microns
385-525 microns 410-525 microns 500 microns 505-720 microns 530-670
microns 555-670 microns
[0018] For larger die in the range of 300 microns and above,
relationship H=1/2Tc+Tb.+-.20 microns may be used. Table II
provides exemplary ranges of H for sample chip thicknesses and
exemplary bond line thickness ranges under this relationship.
TABLE-US-00002 TABLE II Wafer Bond Line Thickness (Tb) Ranges
Thickness (Tc) 25-200 microns 50-150 microns 75-150 microns 300
microns 155-370 microns 180-320 microns 205-320 microns 355 microns
182-398 microns 207-348 microns 232-348 microns 500 microns 255-470
microns 280-420 microns 305-420 microns 600 microns 305-520 microns
330-470 microns 335-470 microns
[0019] Sloped walls 31 with low heights H are typically formed for
thin chips, and are typically made in thin leadframes (.about.100
microns to .about.150 microns) since such thin chips usually
operate at high speed. As such, the height H is usually more than
50% of the thickness of the leadframe, and typically 65% or more.
Sloped walls 31 with heights H of 150 microns or more are typically
made in leadframes of moderate thickness (.about.250 microns), and
thus usually 60% or more of the thickness Tp of the leadframe.
Nonetheless. the above values of H given in the text and in Tables
I and II may be made in leadframes having common leadframe
thicknesses, such 150 microns and 250 microns, with many value of H
being equal to or greater than the leadframe thickness. The values
for height H as provided in the text and in Tables I and II may be
expressed as percentages (rounded to the nearest whole number)
relative to the common leadframe thicknesses of 150 microns and 250
microns. For example, H=155 microns may be expresses as 103% of a
150-micron thick leadframe, and as 62% of a 250-micron thick
leadframe. Those percentages are incorporated herein by reference
so that they may be incorporated into one or more claims of the
application at a later date.
[0020] In preferred uses of leadframe 10, before die 5 is attached
to paddle 20, a controlled amount of adhesive material for layer 40
is disposed on die paddle 20, within opening 36 defined by sloped
wall 31, and/or on the back surface of semiconductor die 5. This,
in combination with the above-described sloped wall 31 of die
paddle 20, provides several advantageous effects. As a first
advantageous effect, the height of sloped wall 31 relative to the
target bond line thickness and die thickness reduces the chances of
the material of layer 40 spreading beyond attachment area 26 and
interfering with the operation of conduction regions 16. This
enables conductive regions 16 to be more closely spaced to die
paddle 20, and enables the creation of a smaller package. As
additional advantageous effect, when die 5 has a metalized back
surface and solder material is used to provide layer 40, the bond
line thickness (BLT) can be controlled to a desired value when die
5 is allowed to float during the solder reflow process. Due to its
surface tension and metal adhesion properties, the reflowed solder
will spread along the back surface of die 5 and along the top
surface of attachment area 26 up to sloped wall 31, and will form a
layer between paddle 20 and die 5 that has a substantially even
thickness. Thus, a desired bond line thickness can be achieved by
disposed a measured amount of solder having a reflowed volume
substantially equal to the desired bond line thickness times the
area of attachment area 26. As an additional advantageous effect,
in the case that die 5 was placed so that a portion of it lies
outside of opening 36, the surface tension and adhesion properties
of the solder act to pull die 5 completely into opening 36 during
the reflow process (with die 5 in a free floating state). This
enables leadframe 10 to be used with die assembly equipment having
less alignment precision.
[0021] Still referring to FIG. 2, after die 5 has been attached to
die paddle 20, a plurality of conductive structures 60, such as
wire bonds, ribbon bonds, tape-automated bonds ("TAB bonds"), etc.,
are attached between conductive regions 6 (e.g., pads) on
semiconductor die 5 and conductive regions 16 (e.g., leads) of
leadframe 10. Conductive structures 60 electrically connect the
devices and/or circuitry of die 5 with the terminals (regions 16)
of package 100. A body of electrically-insulating molding material
70 is provided over the exposed top surface 21 of leadframe 10, die
5, and conductive structures 60 to provide a sealed package.
Material 70 provides mechanical support to conductive structures
60, prevents them from being bent or torn off by external forces,
and, along with leadframe 10, provides a rugged shell for
semiconductor die package 100. The bottom side 22 of leadframe 10,
which includes the bottom sides of conductive regions 16 and die
paddle 20, are not covered by material 70 and are left exposed to
provide the terminals for the package. Package 100 has a leadless
configuration, which means that there are no conductive leads
extending substantially beyond the dimensions of the package. Some
of conductive structures 60 may be used to electrically couple some
conductive regions 6 with die paddle 20 at the paddle's outer edge
(between ridge 30 and the periphery of paddle 20). In this case, a
ground potential is usually coupled to die paddle 20, which may be
done at the bottom surface 22 of die paddle 20.
[0022] In preferred embodiments, leadframe 10 further comprises a
groove 50 disposed on the bottom surface 22 of paddle 20. Groove 50
may be formed opposite to sloped wall 31, and opposite to ridge 30,
and may be formed by stamping. Groove 50 is more clearly shown in
FIG. 3, which is a bottom plan view of leadframe 10 as incorporated
into a package, such as package 100. Groove 50 provides several
advantageous effects, as best illustrated in FIG. 4, which is a
cross-sectional view of package 100 as mounted to a substrate 120.
Substrate 120 may comprise a board, flexible circuit, or the like,
may have additional packages attached thereto, and may be comprised
by a system or may comprise a complete system along with the
packages attached thereto. The back surface 22 of paddle 20 can be
attached to a portion of board 120, such as a conductive region
122, by an adhesive material 140, which may comprise solder. Groove
50 provides room for excess adhesive material 140 to flow into,
which prevents the material from flashing over to other portions of
board 120, such as adjacent conductive regions that are attached to
conductive regions 16 of package 100. As an additional benefit, the
portion of material 140 that enters groove 50 increases the shear
strength against shear forces between package 100 and board 120,
including shear forces caused by temperature differences between
paddle 20 and board 120. This is because groove 50 provides
additional surface area for adhesive material 140 to bond to paddle
20, and because it enables adhesive material 140 to form a rim of
material around attachment area 26 that intersects the shear forces
(which are parallel to the surface of the package's bottom surface)
at a nonparallel angle that ranges between 45 degrees and 90
degrees. The additional surface area also increases bonding
strength. The additional surface area provided by groove 50 does
not require additional surface area of package 100, thus the shear
strength and bonding strength can be increased without increasing
the size of package 100.
[0023] Semiconductor die 5 may include any suitable semiconductor
device. Suitable semiconductor devices may comprise a semiconductor
material such as silicon, and may include vertical or horizontal
devices. Vertical devices have at least an input at one side of the
die and an output at the other side of the die so that current can
flow vertically through the die. Horizontal devices include at
least one input at one side of the die and at least one output at
the same side of the die so that current flows horizontally through
the die. In some preferred embodiments of package 100, the
semiconductor device in semiconductor die 5 preferably comprises a
vertical power transistor. Vertical power transistors include VDMOS
transistors and vertical bipolar transistors. A VDMOS transistor is
a MOSFET that has two or more semiconductor regions formed by
diffusion. It has a source region, a drain region, and a gate. The
device is vertical in that the source region and the drain region
are at opposite surfaces of the semiconductor die. The gate may be
a trenched gate structure or a planar gate structure, and is formed
at the same surface as the source region. Trenched gate structures
are preferred, since trenched gate structures are narrower and
occupy less space than planar gate structures. During operation,
the current flow from the source region to the drain region in a
VDMOS device is substantially perpendicular to the die
surfaces.
[0024] In a first novel die-attachment process according to the
present invention, which has been briefly described above, solder
material is disposed on attachment area 26 and/or a metalized back
surface of a die 5, the die is placed over attachment area 26 and
substantially within the opening 36 defined by sloped wall 31. The
result of examples of these steps is shown in FIG. 5, where a body
of solder 80 in the form of solder paste has been disposed on
attachment area 26 and within the opening 36 defined by sloped wall
31, and where a die 5 with a metalized back surface 8 has been
placed over the body of solder paste 80, and over area 26 and
substantially within the opening 36. As seen in the figure, die 5
is only in rough alignment with sloped wall 31, which may occur
even when using the most accurate multi-die attachment equipment,
with one edge of the die resting near the top of a portion of
sloped wall 31. However, at least a major portion of the die
overlies the attachment area. For leadframe embodiment having a
sloped wall 31 with a height of 100 microns with respect to
attachment area 26 and angle .theta. of inclination with respect to
the attachment area of 120 degrees, opening 36 is approximately 50
microns wider on each side than attachment area 26, which provides
for a misalignment tolerance for die 5 of at least .+-.50 microns
in the X-Y plane. Leadframe embodiments having sloped walls 31 of
greater height provide proportionally greater tolerances (e.g.,
H=200 microns provide a tolerance of .+-.100 microns, etc.). Next,
heat is applied to the solder material to place it in a liquid
state (i.e., to cause it to reflow, or be in a reflowed state), and
semiconductor die 5 is allowed to float on the liquid solder free
from external forces applied by a die-placement tool. The surface
tension and metal-adhesion properties of the reflowed solder bring
the floating die into more precise alignment to the sloped wall.
During this time, vibrations may be applied to the die paddle, or
the die paddle may be moved in a predetermined motion (e.g.,
circular, oscillatory, etc.), to speed the alignment process. Next,
the solder is allowed to cool to a solid state. The result of these
steps is shown in FIG. 6. Because solder paste comprises volatile
components such as solvents, flux, and plasticizers, the volume of
the reflowed body of solder 80' shown in FIG. 6 is smaller than the
volume of the body of solder paste 80 shown in FIG. 5. Conductive
structures 60 may then be attached, and molding material 70
disposed, as previously described.
[0025] The dice for a particular semiconductor chip design have
variations in their dimensions due to variations in the width of
the saws used to cut the dice from the wafers, and due to
mechanical play in the sawing equipment. This variation may be
denoted by the symbol .+-..sigma.. A typical variation .+-..sigma.
in the widths and lengths of dice is approximately .+-.26 microns
(.about..+-.1 mil). The bond line thickness for the package for a
particular semiconductor die having a width W and length L may be
selected so that the width at the top of the solder layer, as
measured at its intersection with sloped wall 31, is in the range
of W.+-..sigma. and the length in the range of L.+-..sigma..
Selecting the width and length to be (W+.sigma.) and (L+.sigma.),
respectively, may lead to a die misalignment of up to +u in each
dimension. Selecting the width and length to be (W-.sigma.) and
(L-.sigma.), respectively, could allow the die have substantially
perfect alignment in at least one of the dimensions (either width
or length) with two of its opposing edges abutting sloped wall 31,
with a misalignment of up to .+-..sigma. in the other
dimension.
[0026] The quantity of solder material is preferably dispensed in a
measured preset amount to provide a finished solder layer having a
reproducible bond line thickness in the range of .about.25 microns
to .about.200 microns, and more preferably in the range of
.about.50 microns to .about.150 microns, and most preferably in the
range of .about.75 microns to .about.150 microns. The solder
material may be dispensed by solder paste dispensing, ribbon
dispensing, and perform dispensing, and other methods providing
measured control. Preferably, the solder material is dispensed so
that the bond line thickness of the finished solder layer is within
5 microns of the target thickness when the method is repeated on
other instances of the die paddle and a die.
[0027] Another novel die-attachment process is shown with reference
to FIGS. 7 and 8. A body of adhesive material 90 in a liquid state
is disposed on attachment area 26 and/or the back surface of a die
5, the die is placed over attachment area 26 and substantially
within the opening 36 defined by sloped wall 31. While in its
liquid state, the form of adhesive material 90 may selected such
that it has sufficient viscosity and surface adhesion that it may
cling to the back of die 5 without dripping off while die 5 is
moved into position over attachment area 26. As seen in FIG. 7, die
5 is only in rough alignment with sloped wall 31, with one edge of
the die resting near the top of a portion of sloped wall 31, but at
least a major portion of the die overlays attachment area 26, with
the adhesive material located between the die and the attachment
area. In some cases, such as when the liquid adhesive material has
a low viscosity and a density greater than that of the die and, die
5 aligns itself to sloped wall 31 shortly after being disposed in
opening 36 and before the adhesive material changes to a solid
state.
[0028] In other cases, the following actions may be preformed while
adhesive material 90 is in its liquid state to bring die 5 into
alignment with sloped wall 31: (1) applying a force in the
direction of gravity to the exposed surface of the die (preferably
by a member with a flat surface, such as a plate) without
substantially restriction the die's horizontal movement relative to
the die paddle, (2) applying vibrations to at least one of the die
paddle and the plate, (3) and moving at least one of the die paddle
and the plate. The first action can be provided by a small plate
210 (generally with an area less than a square inch) that is
lowered over die 5 by a retractable member 212, which is coupled to
plate 210 by flexible members 214 (which may comprise strings,
flexible wires or chains of 2 or more links). When retractable
member 212 is in its raised potion, flexible members 214 are taut
and raise plate 210 so that it does not contact die 5. When
retractable member 212 is in its lowered potion, flexible members
214 are slack and enable plate 210 to contact die 5, and enable
plate 210 and die to move in the horizontal X and Y directions.
Plate 210 preferably weighs between 2 grams and 10 grams, and
applies a downward force substantially equal to its weight. The
second action of applying vibrations may be accomplished by
attaching a piezoelectric transducer to plate 210 and driving it
with an electric signal, and/or coupling and operating such a
transducer to a support member 220 that supports die paddle 20. The
third action of moving may be accomplished by moving one or both of
retractable member 212 and support member 220, such with a circular
motion or an oscillatory motion.
[0029] The semiconductor die packages described above can be used
in electrical assemblies including circuit boards with the packages
mounted thereon. They may also be used in systems such as phones,
computers, etc.
[0030] Preferred examples described above are directed to
"leadless" type packages such as MLP-type packages (microleadframe
packages) where the terminal ends of the leads do not extend past
the lateral edges of the molding material. Embodiments of the
invention may also include leaded packages where the leads extend
past the lateral surfaces of the molding material.
[0031] Any recitation of "a", "an", and "the" is intended to mean
one or more unless specifically indicated to the contrary.
[0032] The terms and expressions which have been employed herein
are used as terms of description and not of limitation, and there
is no intention in the use of such terms and expressions of
excluding equivalents of the features shown and described, it being
recognized that various modifications are possible within the scope
of the invention claimed.
[0033] Moreover, one or more features of one or more embodiments of
the invention may be combined with one or more features of other
embodiments of the invention without departing from the scope of
the invention.
[0034] While the present invention has been particularly described
with respect to the illustrated embodiments, it will be appreciated
that various alterations, modifications, adaptations, and
equivalent arrangements may be made based on the present
disclosure, and are intended to be within the scope of the
invention and the appended claims.
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